LCD devices are commonly used as displays for compact electronic apparatuses. This is because they not only provide good quality images with little power consumption, but also they are very thin. A typical LCD device includes a power supply circuit, which supplies operating voltages for various kinds of working units in the LCD device.
Referring to FIG. 3, a conventional power supply circuit 100 for an LCD device (not labeled) includes a low drop-out linear regulator 110, four filter capacitors 121, 122, 123, 124, and a dividing circuit 130. The low drop-out linear regulator 110 transfers an input voltage Vin from an external circuit to a adjustable or a fixed output voltage Vout, and provides the output voltage Vout to a rear direct current/direct current (DC/DC) converter. The dividing circuit 130 is used to adjust and determine the output voltage from the low drop-out linear regulator 110. The first filter capacitor 121 and the second filter capacitor 122 are parallel connected between the input voltage and ground, for low-pass filtering or high-pass filtering the input voltage Vin. The third filter capacitor 123 and the fourth filter capacitor 124 are parallel connected between the output voltage and ground, for low-pass filtering or high-pass filtering the output voltage Vout.
The dividing circuit 130 has a first resistor 131, a second resistor 132, a shunt capacitor 134 and a dividing node 135. The first and the second resistors 131, 132 are connected in series to ground, defining a series branch. The diving node 135 is disposed between the first and the second resistors 131, 132. The shunt capacitor 134 is connected between the diving node 135 and ground, which can prevent the low drop-out linear regulator 110 from increasing a voltage amplification of the output voltage Vout, and inhibit the voltage ripple of the output voltage Vout.
The low drop-out linear regulator 110 includes a voltage input terminal 112, a voltage output terminal 113, and a voltage adjust terminal 114. The input voltage Vin is transmitted to the voltage input terminal 112 after being filtered by the first and the second filter capacitors 121, 122. The voltage output terminal 113 is connected to one end of the series branch of the dividing circuit 130, and the output voltage Vout is supplied to the rear DC/DC converter after being filtered by the third and the fourth filter capacitors 123, 124. The voltage adjust terminal 114 is connected to the dividing node 135, and defines a feedback loop with the dividing circuit 130. The feedback loop provides a reference voltage Vref to the low drop-out linear regulator 110 and adjust the output voltage Vout thereof. The reference voltage Vref is 1.25V voltage difference between the output terminal 113 and the voltage adjust terminal 114 of the low drop-out linear regulator 110, which is defined by the internal circuits of the low drop-out linear regulator 110.
In operation, the input voltage Vin is provided to the low drop-out linear regulator 110 through the voltage input terminal 112, and is modulation transferred to an idea output voltage Vout transmitting out through the output terminal 113. The output voltage Vout is adjusted through the feedback loop of the voltage adjust terminal 114 and the dividing circuit 130, which substantially equals to Vout=Vref(1+R1/R2), wherein R1 is the resistance value of the first resistor 131, and R2 is the resistance value of the second resistor 132. Thus, the adjustment of the output voltage Vout can be realized through the adjusting of the resistance values of the first and the second resistor 131, 132.
However, when the liquid crystal display (LCD) device operates in a stand-by mode, the DC/DC converter 100 keeps supplying output voltage Vout to the rear DC/DC converter of the power supply circuit of the LCD device. Thus, a large quantity of electric energy loss is produced, which makes the power supply circuit have a overlarge power dissipation.
What is needed, therefore, is a power supply circuit that can overcome the above-described deficiencies.